So this idiom tends to be used within a sentence. For example, you could say: these watches are a dime a dozen, why don’t you get something better?
Another example: nutrition quacks are a dime a dozen these days, why don’t you ask your doctor to recommend someone competent?
Perhaps you’ve noticed, then, that using this expression isn’t exactly complementary. You aren’t referring to the simple ubiquitous quality of an object or service, etc., but to the the fact that you can find it everywhere and not necessarily of good quality, or that it’s so common that it’s uninteresting.
Can you think of any other example sentences?